Population of metal oxide nanosheets, preparation method thereof, and electrical conductor and electronic device including the same

ABSTRACT

An electrical conductor includes a substrate; and a first conductive layer disposed on the substrate and including a plurality of metal oxide nanosheets, wherein adjacent metal oxide nanosheets of the plurality of metal oxide nanosheets contact to provide an electrically conductive path between the contacting metal oxide nanosheets, wherein the plurality of metal oxide nanosheets include an oxide of Re, V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or a combination thereof, and wherein the metal oxide nanosheets of the plurality of metal oxide nanosheets have an average lateral dimension of greater than or equal to about 1.1 micrometers. Also an electronic device including the electrical conductor, and a method of preparing the electrical conductor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.15/342,308, filed Nov. 3, 2016, which claims priority to and the benefitof Korean Patent Application No. 10-2015-0156011, filed in the KoreanIntellectual Property Office on Nov. 6, 2015, and all the benefitsaccruing therefrom under 35 U.S.C. § 119, the entire content of whichare incorporated herein by reference.

BACKGROUND 1. Field

This disclosure relates to a population of metal oxide nanosheets, apreparation method thereof, and an electrical conductor and anelectronic device including the same.

2. Description of the Related Art

An electronic device, such as a flat panel display such as an LCD or LEDdisplay, a touch screen panel, a solar cell, a transparent transistor,and the like includes an electrically conductive thin film or atransparent electrically conductive thin film. It is desirable for amaterial for a transparent electrode to have high light transmittance(e.g., greater than or equal to about 80% in a visible light region) andlow specific resistance (e.g., less than or equal to about 1×10⁻⁴ Ω*cm).Currently available oxide materials include indium tin oxide (ITO), tinoxide (SnO₂), zinc oxide (ZnO), and the like. ITO is a transparentelectrode material and is a transparent semiconductor having a widebandgap of 3.75 eV, and may be manufactured in a large area using asputtering process. However, in terms of application to a flexible touchpanel, or a UD-grade high resolution display, ITO has poor flexibilityand will inevitably cost more due to limited reserves of indium.Therefore, many attempts have been made to replace it.

Recently, a flexible electronic device, e.g., a foldable or bendableelectronic device, has been drawing attention as a next generationelectronic device. Therefore, there is a need for a material havingimproved transparency, relatively high electrical conductivity, andsuitable flexibility, as well as transparent electrode materials.

SUMMARY

An embodiment provides a flexible electrical conductor having improvedelectrical conductivity and improved light transmittance.

Another embodiment provides an electronic device including theelectrical conductor.

Another embodiment provides a population of metal oxide nanosheets foran electrically conductive layer of the electrical conductor.

Another embodiment provides a method of preparing the population ofmetal oxide nanosheets.

In an embodiment, an electrical conductor includes a substrate; and afirst conductive layer disposed on the substrate and including aplurality of metal oxide nanosheets,

wherein adjacent metal oxide nanosheets of the plurality of metal oxidenanosheets contact to provide an electrically conductive path betweenthe contacting metal oxide nanosheets,

wherein the plurality of metal oxide nanosheets include an oxide of Re,V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or acombination thereof, and

wherein the metal oxide nanosheets of the plurality of metal oxidenanosheets have an average lateral dimension of greater than or equal toabout 1.1 micrometers.

The plurality of metal oxide nanosheets may include ruthenium oxide,vanadium oxide, manganese oxide, cobalt oxide, iron oxide, rheniumoxide, iridium oxide, indium oxide, or a combination thereof.

The plurality of metal oxide nanosheets may have an average lateraldimension of greater than or equal to about 1.5 μm and an averagethickness of less than or equal to about 5 nm.

The first conductive layer may be a discontinuous layer including openspaces between the plurality of metal oxide nanosheets, and an arearatio of open spaces relative to the first conductive layer may be lessthan or equal to about 50%.

The first conductive layer may have sheet resistance of less than orequal to about 33000 ohm/sq. at light transmittance of about 93% orgreater. The first conductive layer may have sheet resistance of lessthan or equal to about 15000 ohm/sq. at light transmittance of about 93%or greater.

The metal oxide nanosheet may include RuO_(2+x) (0≤x≤0.5), MnO₂, Mn₃O₇,Mn_(1−x)Co_(x)O₂ (0<x≤0.4), VO₂, CoO₂, FeO₂, ReO₂, IrO₂, InO₂, or acombination thereof.

The electrical conductor may further include a second conductive layerdisposed on the substrate and including an electrically conductive metalnanowire.

The second conductive layer may be disposed between the substrate andthe first conductive layer.

The second conductive layer may be disposed on a surface of the firstconductive layer.

The conductive metal may include silver (Ag), copper (Cu), gold (Au),aluminum (Al), cobalt (Co), palladium (Pd), or a combination thereof.

The electrically conductive metal nanowire may have an average diameterof less than or equal to about 50 nm and an average length of greaterthan or equal to about 1 μm.

The electrical conductor may have transmittance of greater than or equalto about 85% at a wavelength of a 550 nm and sheet resistance of lessthan or equal to about 100 ohm/sq (for example, when the first and/orsecond conductive layers have a thickness of 100 nm or less, e.g., 90 nmor less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 40nm or less, 30 nm or less, or 20 nm or less,).

The electrical conductor may further include an overcoating layerincluding a thermosetting resin, an ultraviolet (UV) curable resin, or acombination thereof directly on the first conductive layer.

The electrical conductor may further include an overcoating layerincluding a thermosetting resin, an ultraviolet (UV) curable resin, or acombination thereof directly on the second conductive layer.

The electrical conductor may have a resistance variation ratio of lessthan or equal to about 60% after being bent at a curvature radius ofabout 1 mm.

Another embodiment provides an electronic device including theelectrical conductor.

The electronic device may be a flat panel display, a touch screen panel,a solar cell, an e-window, an electrochromic mirror, a heat mirror, atransparent transistor, or a flexible display.

Another embodiment provides a population of nanosheets including aplurality of metal oxide nanosheets, wherein the nanosheets include anoxide of Re, V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe,or a combination thereof, wherein an average lateral dimension of theplurality of metal oxide nanosheets is greater than or equal to about1.1 micrometers.

The plurality of metal oxide nanosheets may have an average lateraldimension of greater than or equal to about 1.5 μm and an averagethickness of less than or equal to about 5 nm.

The plurality of metal oxide nanosheets may include ruthenium oxide,vanadium oxide, manganese oxide, cobalt oxide, iron oxide, rheniumoxide, iridium oxide, indium oxide, or a combination thereof.

The metal oxide nanosheet may include RuO_(2+x) (0≤x≤0.5), MnO₂, Mn₃O₇,Mn_(1−x)Co_(x)O₂ (0<x≤0.4), VO₂, CoO₂, FeO₂, ReO₂, IrO₂, InO₂, or acombination thereof.

Another embodiment provides a method of preparing the population of thenanosheets, the method including

heat-treating a mixture including a transition metal oxide including Re,V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or acombination thereof and an alkali metal compound at a temperature ofabout 750° C. to about 950° C. for about 18 hours or more to obtain alayered alkali metal-transition metal oxide;

pulverizing the layered alkali metal-transition metal oxide to obtain apowder of the layered alkali metal-transition metal oxide;

rinsing the powder of the layered alkali metal-transition metal oxidewith water to obtain a powder of a layered alkali metal-transition metaloxide hydrate;

treating the powder of the layered alkali metal-transition metal oxidehydrate with an acidic solution to obtain a layered proton-exchangedtransition metal oxide hydrate wherein at least a portion of an alkalimetal is exchanged with a proton;

contacting the layered proton-exchanged transition metal oxide hydratewith a C1 to C16 alkyl ammonium salt compound to obtain a layeredtransition metal oxide intercalated with a C1 to C16 alkyl ammoniumcation; and mixing the layered transition metal oxide intercalated witha C1 to C16 alkyl ammonium cation with a solvent to obtain a populationof transition metal oxide nanosheets.

The heat-treating may be performed for about 24 hours or longer.

The powder of the layered alkali metal-transition metal oxide hydratemay have an average particle diameter of greater than or equal to about100 μm.

The electrical conductor of the aforementioned embodiments has anelectrically conductive layer including nanosheets with an increasedlateral dimension and thus decreased contact resistance between thenanosheets. Therefore, the electrical conductor of the embodiments mayhave an enhanced conductivity at a high level of light transmittance andimproved flexibility.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic view of an electrical conductor according to oneembodiment;

FIG. 2 is a cross-sectional schematic view of an electronic device(touch screen panel) according to one embodiment;

FIG. 3 is a schematic view showing a random work path in atwo-dimensional percolation simulation of the nanosheets that isconducted in in Example 1;

FIG. 4 shows a sheet resistance change with respect to the fixed arealcoverage obtained by two-dimensional percolation calculation ofnanosheets;

FIG. 5 is a schematic view showing a process of preparing rutheniumoxide nanosheets according to the Examples;

FIG. 6 shows a heat treatment profile for synthesizing an alkalimetal-ruthenium oxide in the Examples;

FIG. 7 shows an X-ray diffraction analysis spectrum of the alkalimetal-ruthenium oxide hydrates obtained from Examples 1 to 3 andComparative Example 1;

FIG. 8 is a scanning electron microscope image of alkali metal-rutheniumoxide hydrate obtained from Example 1;

FIG. 9 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Example 1;

FIG. 10 is an X-ray diffraction spectrum of ruthenium oxide nanosheetsobtained from Example 1 and Example 4;

FIG. 11 is a scanning electron microscope image of alkalimetal-ruthenium oxide hydrate obtained from Example 2;

FIG. 12 shows XPS analysis results of alkali metal-ruthenium oxidehydrate obtained Example 2 and the proton-exchanged powder;

FIG. 13 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Example 2;

FIG. 14 is a scanning electron microscope image of alkalimetal-ruthenium oxide hydrate obtained from Example 3;

FIG. 15 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Example 3;

FIG. 16 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Example 4;

FIG. 17 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Example 5;

FIG. 18 is a scanning electron microscope image of alkalimetal-ruthenium oxide hydrate obtained from Comparative Example 1;

FIG. 19 is a scanning electron microscope image of ruthenium oxidenanosheets obtained from Comparative Example 1;

FIG. 20 is a graph showing a sheet resistance with respect to an averagelateral dimension of conductive layers obtained from Examples 6 to 10and Comparative Example 2;

FIG. 21 is a view schematically illustrating a method of performing aflexibility evaluating test (computer simulation) of Example 12;

FIG. 22 shows the flexibility evaluating test (simulation) results ofExample 12;

FIG. 23 shows the X-ray diffraction analysis results of NaVO₂ obtainedfrom Reference Example 2;

FIG. 24 is a schematic view showing a C12/M1 (12) crystal structure ofAMO₂ metal (A: alkali metal, M: vanadium or manganese);

FIG. 25 is a schematic view showing a R3MH (166) crystal structure ofAMO₂ metal (A: alkali metal, M: ruthenium or cobalt).

DETAILED DESCRIPTION

Advantages and characteristics of this disclosure, and a method forachieving the same, will become evident referring to the followingexemplary embodiments together with the drawings attached hereto.However, this disclosure may be embodied in many different forms and isnot be construed as limited to the embodiments set forth herein. If notdefined otherwise, all terms (including technical and scientific terms)in the specification may be defined as commonly understood by oneskilled in the art. The terms defined in a generally-used dictionary maynot be interpreted ideally or exaggeratedly unless clearly defined. Inaddition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising,” will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements.

Further, the singular includes the plural unless mentioned otherwise.

In the drawings, the thickness of layers, regions, etc., are exaggeratedfor clarity. Like reference numerals designate like elements throughoutthe specification.

It will be understood that when an element such as a layer, film,region, or substrate is referred to as being “on” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.

“At least one” is not to be construed as limiting “a” or “an.” “Or”means “and/or.” As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. It will befurther understood that the terms “comprises” and/or “comprising,” or“includes” and/or “including” when used in this specification, specifythe presence of stated features, regions, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, regions, integers, steps,operations, elements, components, and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

As used herein, the sheet resistance refers to a value defined by the4-point probe measurement for the specimen having a predetermined size(e.g., a width of about 8 centimeters (cm) and a length of about 8 cm).

As used herein, the term “nanosheets” refers to a two-dimensional nanomaterial having a thickness in a nano-order size (e.g., thickness ofless than or equal to about 100 nanometers (nm)).

“The nanosheets being in contact with one another to provide anelectrical connection (e.g., the electrically conduction path)” mayrefer to the case where the contact between the nanosheets is made toprovide an electrical conduction path, and thereby the conductive layerhas an electrical conductivity (for example, of a sheet resistance ofless than or equal to about 1,000,000 ohm/sq.)

An electrical conductor of an embodiment, e.g., as shown in FIG. 1,includes a substrate 10; and a first conductive layer including aplurality of metal oxide nanosheets 11 disposed on the substrate and theplurality of metal oxide nanosheets include an electrically conductivepath 12 provided by contacting metal oxide nanosheets. The plurality ofmetal oxide nanosheets may include ruthenium oxide, vanadium oxide,manganese oxide, cobalt oxide, or a combination thereof. The pluralityof metal oxide nanosheets may have an average lateral size, i.e., anaverage lateral dimension, of greater than or equal to about 1.1 μm. Theterms “providing an electrically conductive path” refers to the casewhereby the first conductive layer has an electrical conductivity in aplane direction.

The substrate may be a transparent substrate. The substrate may beflexible. A material for the substrate is not particularly limited, andthe substrate may be a glass substrate, a semiconductor substrate, apolymer substrate, or a combination thereof, it may be a substrate inwhich an insulation layer and/or an electrically conductive film are/islaminated. As non-limiting examples, the substrate may include aninorganic material such as glass; polyester such as polyethyleneterephthalate, polybutylene terephthalate, and polyethylene naphthalate;polycarbonate; an acryl-based resin; cellulose or a derivative thereof;a polymer such as polyimide; an organic/inorganic hybrid material; or acombination thereof. The thickness of the substrate is also notparticularly limited, but may be appropriately selected considering thetype of the final product. The substrate may have a thickness of greaterthan or equal to about 0.5 μm, for example, greater than or equal toabout 1 μm, greater than or equal to about 10 μm, but is not limitedthereto. The substrate may have a thickness of less than or equal toabout 1 mm, for example, less than or equal to about 500 μm, or lessthan or equal to about 200 μm, but is not limited thereto. Additionallayer (e.g., undercoat) may be provided between the substrate and theelectrically conductive layer if desired (e.g., in order to control arefractive index).

A first conductive layer including a plurality of metal oxide nanosheetsis disposed on the substrate. The plurality of metal oxide nanosheetsinclude an electrical connection between contacting metal oxidenanosheets. The amount of open space in the first conductive layer isdetermined by measured an area of the open space and comparing to atotal area of the first conductive layer. For example, a ScanningElectron Microscopic image of the first conductive layer includingnanosheets disposed to have an open space is obtained and the area ofthe open space (i.e., the portion not having the nanosheets in the firstconductive layer) is determined and is divided with the total area ofthe first conductive layer to provide an area ratio. The firstconductive layer may be a discontinuous layer including open spacesbetween the plurality of metal oxide nanosheets, and an area ratio ofopen spaces relative to a total area of the first conductive layer maybe less than or equal to about 50%, for example, less than or equal toabout 40%, or less than or equal to about 30%.

The plurality of metal oxide nanosheets includes an oxide of a metalselected from Re, V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co,Fe and a combination thereof. The metal oxide may include rutheniumoxide, vanadium oxide, manganese oxide, cobalt oxide, or a combinationthereof. In an embodiment, the plurality of metal oxide nanosheets mayinclude ruthenium oxide, vanadium oxide, manganese oxide, cobalt oxide,iron oxide, rhenium oxide, iridium oxide, indium oxide, or a combinationthereof. For example, in the plurality of metal oxide nanosheets, themetal oxide may include RuO_(2+x) (0≤x≤0.5, for example, 0≤x≤0.1), MnO₂,Mn₃O₇, Mn_(1−x)Co_(x)O₂ (0<x≤0.4), VO₂, CoO₂, FeO₂, ReO₂, IrO₂, InO₂, ora combination thereof.

In the case of the metal oxide, the simulation may provide theelectrical conductivity, the light absorption coefficient, and the sheetresistance, and the material may have a high conductivity and a lowabsorption coefficient to be applied for a transparent electrodematerial. For example, processing the layered oxide bulk material intonanosheets having a thin thickness (less than or equal to several tensnm) may allow to ensure the high transmittance in a visible lightregion.

The simulation is based on a first principles calculation. The firstprinciples calculation is a simulation method made by the quantummechanics and may calculate the optimized atomic structure of materialand the electronic structure according to the same. The electronicstructure of material is a reference quantity for calculating anelectrical conductivity and a light absorption coefficient of thematerial.

First, it may obtain an optimized atomic structure of MO₂-type (M=Re, V,Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, or Fe) including thevarious transition metals, and the structure, for example, shown in FIG.24 and FIG. 25 may be considered.

Subsequently, a conductivity and an absorption coefficient of MO₂-typemetal oxide are calculated according to the following simulation steps,and with reference to the results, the transmittance and the sheetresistance based on 98% may be calculated.

The band-structure of MO2 type material is calculated according toFirst-principles electronic structure calculations. The intra-bandtransition by free electron is calculated from the band structure toobtain a conductivity and a plasma frequency.

An inter-band transition due to bound electron is calculated from theband structure.

Dielectric function is calculated considering the effects of freeelectron and bound electron.

The square-root of dielectric function is obtained to calculate arefractive function in a complex number.

A refractive index to the visible light and the absorption rate for thevisible light are calculated from the refractive function.

Table 1 shows the simulation results of calculating a conductivity andan absorption coefficient of the MO₂ type (M=metal) material.Considering an absorption coefficient (α) and a resistivity (ρ) forlight having a wavelength of 550 nm at a room temperature, α×ρ iscalculated; and the sheet resistance (Ω/sq) considering a transmittanceof 98% is calculated for the corresponding material. In addition, toanticipate the synthesis possibility, the heat of formation (Hf) is alsocalculated.

TABLE 1 Hf σ_xy (S/cm) α_xy αρ_xy Rs_xy @98% (eV/f.u.) (T = 10⁻¹⁴)(1/cm) (Ω/▭) (Ω/▭) Re₁O₂ −3.21 1.00 · 10⁵ 4.8 · 10⁴ 0.5 23.8 V₁O₂ −6.493.07 · 10⁴ 4.1 · 10⁴ 1.3 66.8 Os₁O₂ −1.62 6.70 · 10⁴ 1.1 · 10⁵ 1.7 83.2Ru₁O₂ −2.19 3.55 · 10⁴ 6.010⁴ 1.7 83.7 Ta₁O₂ −7.01 4.85 · 10⁴ 8.6 · 10⁴1.8 88.2 Ir₁O₂ −1.71 3.85 · 10⁴ 7.8 · 10⁴ 2.0 100.2 Nb₁O₂ −6.89 3.82 ·10⁴ 1.0 · 10⁵ 2.7 134.6 W₁O₂ −4.64 5.32 · 10⁴ 1.8 · 10⁵ 3.4 169.6 Ga₁O₂−3.08 2.11 · 10⁴ 9.0 · 10⁴ 4.3 210.6 Mo₁O₂ −4.67 4.42 · 10⁴ 1.9 · 10⁵4.3 215.2 In₁O₂ −2.42 2.24 · 10⁴ 1.0 · 10⁵ 4.6 227.7 Cr₁O₂ −4.74 1.51 ·10⁴ 8.1 · 10⁴ 5.4 265.6 Rh₁O₂ −1.89 3.10 · 10⁴ 1.7 · 10⁵ 5.6 276.1 Mn₁O₂−3.93 1.95 · 10⁴ 1.2 · 10⁵ 6.1 299.8

With reference to the results of Table 1, the metal oxides may show arelatively low sheet resistance (e.g., 300 ohm/sq) at an improvedtransmittance.

An average thickness of the metal oxide nanosheet may be less than orequal to about 5 nm, for example, less than or equal to about 3 nm, lessthan or equal to about 2.5 nm or less than or equal to about 2 nm. Theaverage thickness of the metal oxide nanosheet may be greater than orequal to about 1 nm, for example, greater than 1 nm. When the averagethickness is less than or equal to about 3 nm, light transmittance maybe improved.

An average lateral size of the plurality of metal oxide nanosheets maybe greater than or equal to about 1.1 μm, for example, greater than orequal to about 1.2 μm, greater than or equal to about 1.3 μm, greaterthan or equal to about 1.4 μm, or greater than or equal to about 1.5 μm.The average lateral size of the plurality of metal oxide nanosheets maybe less than or equal to about 30 μm, for example, less than or equal toabout 20 μm, less than or equal to about 15 μm, less than or equal toabout 14 μm, less than or equal to about 13 μm, less than or equal toabout 12 μm, less than or equal to about 11 μm, or less than or equal toabout 10.5 μm, but is not limited thereto. The metal oxide nanosheetshaving the lateral size within the range may provide a conductive layerin the relatively easy way, and the obtained conductive layer may showan improved conductivity at a relatively high light transmittance.

In the conventional art using the intercalation and the exfoliation, themetal oxide nanosheets have an average lateral size in a sub-micronorder. When the metal oxide nanosheets having a lateral size ofsub-micron (e.g., several hundreds nanometer-order) are connected toeach other to provide a conductive path, the number of contact portionsrequired for a unit area is significantly many, causing the high contactresistance. Accordingly, comparing to the sheet resistance which isestimated that single nanosheet may accomplish, the sheet resistance ofthe conductive layer including these nanosheets may be remarkablyenhanced. On the contrary, in the electrical conductor according to anembodiment, as metal oxide nanosheets having a significantly increasedsize (e.g., average lateral size of greater than or equal to approximate1.1 μm) form a first conductive layer, the resultant electricalconductor may show a decreased sheet resistance and an improvedelectrical conductivity at a predetermined light transmittance. In anembodiment, the first conductive layer may have sheet resistance of lessthan or equal to about 33000 ohm/sq., for example 32000 ohm/sq., lessthan or equal to about 30000 ohm/sq., less than or equal to about 29000ohm/sq., less than or equal to about 28000 ohm/sq., less than or equalto about 27000 ohm/sq., less than or equal to about 26000 ohm/sq., lessthan or equal to about 25000 ohm/sq., less than or equal to about 24000ohm/sq., less than or equal to about 23000 ohm/sq., less than or equalto about 22000 ohm/sq., less than or equal to about 21000 ohm/sq., lessthan or equal to about 20000 ohm/sq., less than or equal to about 19000ohm/sq., less than or equal to about 18000 ohm/sq., less than or equalto about 17000 ohm/sq., less than or equal to about 16000 ohm/sq., lessthan or equal to about 15000 ohm/sq., less than or equal to about 14000ohm/sq., less than or equal to about 13000 ohm/sq., less than or equalto about 12500 ohm/sq., less than or equal to about or 12000 ohm/sq. atlight transmittance of about 93% or greater.

The metal oxide nanosheets having the thickness and lateral size may beprepared by a method including the steps:

heat-treating a mixture including transition metal oxide including Re,V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or acombination thereof and an alkali metal compound at a temperature ofabout 750° C. to about 950° C. for about 18 or more to obtain a layeredalkali metal-transition metal oxide;

pulverizing the layered alkali metal-transition metal oxide to obtain apowder of the layered alkali metal-transition metal oxide;

rinsing the powder of the layered alkali metal-transition metal oxidewith water to obtain a powder of a layered alkali metal-transition metaloxide hydrate;

treating a powder of the layered alkali metal-transition metal oxidehydrate with an acidic solution to obtain a layered proton-exchangedtransition metal oxide hydrate where at least a portion of an alkalimetal is exchanged with a proton;

contacting the layered proton-exchanged transition metal oxide hydratewith a C1 to C16 alkyl ammonium salt compound to obtain a layeredtransition metal oxide intercalated by alkyl ammonium; and

mixing the layered transition metal oxide intercalated by alkyl ammoniumwith a solvent to obtain a population of transition metal oxidenanosheets.

Examples of the transition metal oxide may be RuO₂, VO₂, MnO₂, CoO₂,FeO₂, ReO₂, IrO₂, InO₂, ReO₂, OsO₂, TaO₂, NbO₂, WO₂, GaO₂, MoO₂, CrO₂,RhO₂, or a combination thereof, but are not limited thereto, andexamples of the alkali metal compound may be alkali metal carbonate(e.g., Na₂CO₃, K₂CO₃, Li₂CO₃, Cs₂CO₃ etc.), but are not limited thereto.The mixing ratio of between the transition metal oxide and the alkalimetal compound may be appropriately selected considering the compositionof metal oxide to be prepared. For example, about 0.1 mol to about 1 molof alkali metal compound may be mixed per 1 mol of transition metaloxide, but is not limited thereto.

The obtained mixture is heated under the inert atmosphere (e.g.,nitrogen atmosphere, argon atmosphere, or vacuum) at a temperature ofabout 750° C. to about 950° C. (e.g., about 800° C. to about 950° C.)for greater than or equal to about 18 hours, for example, greater thanor equal to about 22 hours, greater than or equal to about 23 hours, orgreater than or equal to about 24 hours) to provide a layered alkalimetal-transition metal oxide. They have a layered structure as shown inFIG. 24, wherein M is an alkali metal, O is oxygen, and T is atransition metal. When is described with an example of alkali metalruthenium oxide, the alkali metal-ruthenium oxide may have a layeredstructure of M-RuO₂-M-RuO₂-M (M=Li, Na, K) (e.g., R3MH (166)).Similarly, the alkali metal-vanadium oxide MVO₂ (M=Li, Na, K) may have alayered C12/M1 (12) crystal structure. The alkali metal-manganese oxideMMnO₂ (M=Li, Na, K) may have a layered C12/M1 (12) crystal structure(reference: FIG. 24). The alkali metal-cobalt oxide MCoO₂ (M=Li, Na, K)may have a layered R3MH (166) structure (reference: FIG. 25). The alkalimetal-transition metal oxide may have a grain size increased under theheat treatment condition.

The obtained layered alkali metal-transition metal oxide is pulverizedto provide powder of the layered alkali metal-transition metal oxide,and the obtained powder is rinsed with water to provide powder oflayered alkali metal-transition metal oxide hydrate. By the rinsing, theexcessive amount of water-soluble components such as alkali metal saltis removed to provide powder including sheet-shaped particles of layeredalkali metal-transition metal oxide hydrate.

Under the heat treatment condition, as the sheet-shaped grain size ofthe layered alkali metal-transition metal oxide is increased, thenanosheets obtained from the later-described exfoliation step may havean increased lateral size. In addition, the heat treatment condition maysuppress the forming a secondary phase. Accordingly, an average particlesize of the powder of the layered alkali metal-transition metal oxidehydrate may be greater than or equal to about 100 μm, for example,greater than or equal to about 110 μm, greater than or equal to about120 μm, greater than or equal to about 130 μm, greater than or equal toabout 140 μm, greater than or equal to about 150 μm, greater than orequal to about 160 μm, greater than or equal to about 170 μm, greaterthan or equal to about 180 μm, greater than or equal to about 190 μm, orgreater than or equal to about 200 μm. When the powder of the layeredalkali metal-transition metal oxide hydrate has the size, the resultantnanosheets (by intercalation and exfoliation) may have a lateral size ofgreater than or equal to about 1.1 μm.

When the powder of the layered alkali metal-transition metal oxidehydrate is treated with an acidic solution (e.g., agitated in an acidicaqueous solution such as hydrochloric acid, sulfuric acid and the like),a layered proton-exchanged transition metal oxide hydrate that at leasta part of alkali metal (e.g., almost or all alkali metal) is exchangedwith proton (H⁺) may be obtained. The acid concentration of acidicaqueous solution, the treatment temperature, the treatment time, and thelike may be appropriately selected, and are not particularly limited.The ion exchange process does not substantially change the sheet-shapedmorphology of the oxide hydrate and does not cause the size decrease.The forming nanoparticles may be suppressed in theseparation/purification process.

The obtained layered proton-exchanged transition metal oxide hydratecontacts a C1 to C16 alkylammonium salt compound (e.g., an aqueoussolution of alkylammonium salt compound) (hereinafter, referred tointercalation treatment) to provide a layered transition metal oxide inwhich alkyl ammonium is intercalated (hereinafter, referred to layeredalkylammonium-transition metal oxide). The alkylammonium salt moleculeis entered between metal oxide layers to help the exfoliation.

The concentration of an aqueous solution of alkylammonium salt compoundmay be about 0.01 to about 20 mol % based on proton of the layeredproton-exchanged transition metal oxide hydrate, but is not limitedthereto. The temperature and time of intercalation treatment are notparticularly limited and may be appropriately selected. For example, theintercalation treatment may be performed at a temperature of about 25°C. to about 60° C. (e.g., room temperature) for greater than or equal toabout 24 hours, for example, greater than or equal to about 2 days orgreater than or equal to about 3 days, but is not limited thereto.

According to an embodiment, the alkyl ammonium salt compound may be amixture of two or more kinds of compounds having different sizes. Whenthe intercalation treatment is performed by using two or more kinds ofalkyl ammonium salt compounds, the two or more kinds of alkyl ammoniumsalt compounds (hereinafter, also called ‘intercalant’) may be presenton the surface of the obtained metal oxide nanosheets.

When the small-sized intercalant is used together with and thelarge-sized intercalant, the small intercalant is easily intercalatedbetween layers of layered protonated transition metal oxide hydrate,which may help the large intercalant to uniformly intercalate betweenlayers of metal oxide hydrate. Accordingly, compared to the exfoliationusing the single intercalant, the case of using two or more kinds ofintercalants may provide nanosheets having an appropriate thickness by ahigh exfoliation efficiency, and the lateral size of the obtainednanosheets is also increased.

The two kinds of alkylammonium compounds may be selected fromtetramethylammonium compound (e.g., tetramethylammonium hydroxide),tetraethylammonium compound (e.g., tetraethylammonium hydroxide),tetrapropylammonium compound (e.g., tetrapropylammonium hydroxide),benzylalkylammonium compound (e.g., benzylmethylammonium hydroxide), andtetrabutylammonium compound (e.g., tetrabutylammonium hydroxide). Thetwo kinds of alkylammonium compounds may include at least one oftetramethylammonium compound and tetraethylammonium compound, and atleast one of a tetrapropylammonium compound, a benzylalkylammoniumcompound and a tetrabutylammonium compound.

When the obtained layered alkylammonium-transition metal oxide is mixedwith a solvent, it is exfoliated to provide nanosheets of transitionmetal oxide. The intercalation treatment and the solvent mixing step maybe simultaneously performed. For the exfoliation, ultrasonication may beperformed. The solvent may be a high dielectric constant solvent. Thesolvent may be one or more selected from water, alcohol, acetonitrile,dimethylsulfoxide, dimethyl formamide, and propylenecarbonate.

The metal oxide nanosheets prepared according to the method has thethickness and the lateral size, and the first conductive layer includingthe same has lowered contact resistance and improved electricalconductivity (i.e., lowered sheet resistance).

The electrical conductor may further include a second conductive layerdisposed on the substrate and including an electrically conductive metalnanowire 13. The second conductive layer may be disposed between thesubstrate and the first conductive layer. Alternately, the firstconductive layer may be disposed on the substrate, and the secondconductive layer may be disposed on the first conductive layer (refer toFIG. 1). The second conductive layer and the first conductive layercontact each other.

The first conductive layer may include a conductive metal such as silver(Ag), copper (Cu), gold (Au), aluminum (Al), cobalt (Co), palladium(Pd), or a combination thereof (e.g., an alloy thereof, or a nanometalwire having two or more segments). For example, the electricallyconductive metal nanowire may be a silver nanowire.

The electrically conductive metal nanowire may have an average diameterof less than or equal to about 50 nm, for example, less than or equal toabout 40 nm, less than or equal to about 30 nm. The length ofelectrically conductive metal nanowire is not particularly limited, butmay be appropriately selected according to a diameter. For example, theelectrically conductive metal nanowire may have a length of greater thanor equal to about 1 μm, greater than or equal to about 2 μm, greaterthan or equal to about 3 μm, greater than or equal to about 4 μm,greater than or equal to about 5 μm, but is not limited thereto. Inanother embodiment, the electrically conductive metal nanowire may havea length of greater than or equal to about 10 μm, for example, greaterthan or equal to about 11 μm, greater than or equal to about 12 μm,greater than or equal to about 13 μm, greater than or equal to about 14μm, or greater than or equal to about 15 μm. The electrically conductivemetal nanowire may be prepared according to known methods, orcommercially available. The nanowire may include a polymer coating ofpolyvinylpyrrolidone on the surface thereof.

The first conductive layer including the metal oxide nanosheets and thesecond conductive layer including the electrically conductive metalnanowire may be formed by the known method of forming a layer, and isnot particularly limited.

According to non-limiting example, a first conductive layer includingmetal oxide nanosheets may be formed on one surface of substrate; and asecond conductive layer including electrically conductive metalnanowires may be formed on one surface of the first conductive layer.Alternatively, a second conductive layer including electricallyconductive metal nanowires may be formed on one surface of substrate;and a first conductive layer including metal oxide nanosheets may beformed on the second conductive layer.

The first conductive layer or the second conductive layer may be formedby applying an appropriate coating composition (including nanosheets ornanowires) on a substrate (or the second or first conductive layer) andremoving a solvent. The coating composition may include an appropriatesolvent (e.g., water, an organic solvent miscibile or non-miscibile withwater, etc.) and may further include a dispersing agent (e.g.,hydroxypropylmethyl cellulose).

For example, the ink composition including the electrically conductivemetal nanowires may be commercially available or may be preparedaccording to known methods. For example, the ink composition may havethe composition shown in Table 2, but is not limited thereto.

TABLE 2 Material Amount Electrically Electrically conductive metal (e.g.Ag)  5-40 wt % conductive nanowire aqueous solution metal(concentration: 0.001-10.0 wt %) Solvent Water 20-70 wt % Alcohol(ethanol) 10-40 wt % Dispersing hydroxypropyl methyl cellulose aqueous 1-10 wt % agent solution (conc.: 0.05-5 wt %)

For example, the composition including the metal oxide nanosheets mayhave the composition shown in Table 3, but is not limited thereto:

TABLE 3 Material Amount Electrically metal oxide nanosheet (e.g.,RuO_(2+x)) aqueous 30-70 wt % conductive solution (concentration:0.001-10.0 g/L) metal Solvent water 10-50 wt % isopropanol  1-20 wt %Dispersing hydroxypropyl methyl cellulose aqueous  5-30 wt % agentsolution (0.05-5 wt %)

The concentration of nanosheet aqueous solution may be appropriatelyadjusted considering a size of nanosheets, a thickness, and theelectrical conductivity required for the electrical conductor, the lighttransmittance required for the electrical conductor, the coatingproperty of an aqueous solution, and the like. For example, theconcentration of nanosheet aqueous solution may be greater than or equalto about 0.001 g/L and less than or equal to about 10.00 g/L, but is notlimited thereto.

The composition is coated on a substrate (or selectively, preliminarilyformed first or second conductive layer) and, selectively, dried andheat-treated to prepare an electrically conductive layer. Thecomposition may be applied according to various methods, for example,bar coating, blade coating, slot die coating, spray coating, spincoating, gravure coating, inkjet printing, or a combination thereof. Thenanosheets may contact each other to provide an electrical connection.

The first conductive layer and/or the second conductive layer mayinclude an organic binder for biding the nanowires or nanosheets. Thebinder desirably controls viscosity of a composition for forming anelectrically conductive layer and increases binding the nanowires on asubstrate. Non-limiting examples of the binder may include methylcellulose, ethyl cellulose, hydroxypropyl methyl cellulose (HPMC),hydroxypropyl cellulose (HPC), xanthan gum, polyvinyl alcohol (PVA),polyvinyl pyrrolidone (PVP), carboxylmethyl cellulose, hydroxyethylcellulose, or a combination thereof. An amount of the binder may beappropriately selected and is not particularly limited thereto. Innon-limiting examples, amount of the binder may be about 1 to about 100parts by weight based on 100 parts by weight of the nano-sizedconductors, but is not limited thereto.

The thickness of first conductive layer is not particularly limited, andmay be appropriately selected considering the light transmittance andthe electrical conductivity required for the electric conductor. Forexample, the thickness of first conductive layer may be less than orequal to about 20 nm, for example, less than or equal to about 5 nm, butis not limited thereto. The thickness of second conductive layer may beless than or equal to about 200 nm, for example, less than or equal toabout 100 nm, but is not limited thereto.

The electrical conductor having the structure may show an enhancedconductivity and an enhanced light transmittance and also show animproved flexibility. The electrical conductor may have lighttransmittance of about 85% or greater for example about 88% or greateror about 89% or greater at a wavelength of a 550 nm and sheet resistanceof less than or equal to about 100 ohm/sq., for example, 90 ohm/sq.,less than or equal to about 80 ohm/sq., less than or equal to about 70ohm/sq., less than or equal to about 60 ohm/sq., less than or equal toabout 50 ohm/sq., less than or equal to about 40 ohm/sq., less than orequal to about 39 ohm/sq., less than or equal to about 38 ohm/sq., lessthan or equal to about 37 ohm/sq., less than or equal to about 36ohm/sq., less than or equal to about or 35 ohm/sq.

The electrical conductor may further include an overcoating layer (OCL)including a thermosetting resin, an ultraviolet (UV) curable resin, or acombination thereof on the first conductive layer or the secondconductive layer. Specific examples of the thermosetting resin and theultraviolet (UV) curable resin for OCL are known. In an embodiment, thethermosetting resin and the ultraviolet (UV) curable resin for anovercoating layer (OCL) may be urethane(meth)acrylate, perfluoropolymerhaving a (meth)acrylate group, poly(meth)acrylate having a(meth)acrylate group, epoxy(meth)acrylate, or a combination thereof. Theovercoating layer may further include an inorganic oxide particulate(e.g., silica particulate). The method of forming the OCL on theelectrically conductive thin film from the above-mentioned materials isalso known, and is not particularly limited.

The electrical conductor may have an improved flexibility. For example,the resistance decreasing rate after bending may be significantly lowerthan the case of using only nanowire. According to an embodiment, theelectrical conductor may have a resistance variation ratio of less thanor equal to about 60%, for example, less than or equal to about 50%,less than or equal to about 40%, or less than or equal to about 30%after bending 200,000 times at a curvature radius of about 1 mm (1R).

In another embodiment, an electronic device includes the electricalconductor.

The electronic device may be a flat panel display, a touch screen panel,a solar cell, an e-window, an electrochromic mirror, a heat mirror, atransparent transistor, or a flexible display.

In an exemplary embodiment, the electronic device may be a touch screenpanel (TSP). The detailed structure of the touch screen panel is wellknown. The schematic structure of the touch screen panel is shown inFIG. 2. Referring to FIG. 2, the touch screen panel may include a firsttransparent conductive film 21, a first transparent adhesive film (e.g.,an optically clear adhesive (OCA)) film 22, a second transparentconductive film 23, a second transparent adhesive film 24, and a windowfor a display device 25, on a panel for a display device (e.g., an LCDpanel) 20. The first transparent conductive layer and/or the secondtransparent conductive layer may be the electrical conductor or hybridstructure.

In addition, an example of applying the electrical conductor to a touchscreen panel (e.g., a transparent electrode of TSP) is illustrated, butthe conductor may be used as an electrode for other electronic devicesincluding a transparent electrode without a particular limit. Forexample, the conductor may be applied as a pixel electrode and/or acommon electrode for a liquid crystal display (LCD), an anode and/or acathode for an organic light emitting diode device, or a displayelectrode for a plasma display device.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. These examples, however, are not in any sense tobe interpreted as limiting the scope of this disclosure.

EXAMPLES Measurement:

[1] Measurement of sheet resistance: Sheet resistance is measured asfollows.

Measurer: Mitsubishi Ioresta-GP (MCP-T610), ESP type probes (MCP-TP08P)

Sample size: width 20 cm×length 30 cm

Measurement: average after repeating the measurement at least 9 times

[2] Measurement of light transmittance: Light transmittance is measuredas follows.

Measurer: NIPPON DENSHOKU INDUSTRIES (NDH-7000 SP)

Sample size: width 20 cm×length 30 cm

Sample Measurement: average after repeating the measurement at least 9times

[3] Measurement of haze: Haze is measured as follows.

Measurer: NIPPON DENSHOKU INDUSTRIES (NDH-7000 SP)

Sample size: width 20 cm×length 30 cm

Sample Measurement: average after repeating the measurement at least 9times

[4] Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM)Analysis: Scanning electron microscope and atom atomic force microscopeanalysis are performed using the following devices to measure a lateralsize and a thickness of nanosheets, a thickness of conductive layers,and the like.

Electron microscope: FE-SEM (Field Emission Scanning ElectronMicroscopy) Hitachi (SU-8030)

Atomic force microscope (SPM): Bruker (Icon)

[5] XPS Analysis: (performed for powders)

Measurer: Manufacturer+Model name

Manufacturer: Ulvac PHI, Model name: Versaprobe

[6] ICP Analysis: (performed for powders)

Measurer: Manufacturer+Model name

Manufacturer: SHIMADZU, Model name: ICPS-8100

Reference Example 1: Two-dimensional Percolation Calculation

In the conductive layer including metal oxide nanosheets, in order tofind the relationship between the lateral size of metal oxide nanosheetsand the conductivity of conductive layer, RuO_(2+x) nanosheets areperformed with a 2D percolation calculation as follows:

RuO_(2+x) is simplified as a disk having a diameter of average lateralsize of nanosheets, and the lateral conductivity (σ_(lateral direction))of RuO_(2+x) is obtained from First principles calculation. For theconvenience of calculation, a contact resistance (Rc) between sheets isapproximated through the vertical direction conductivity(σ_(thickness direction)) of RuO_(2+x) nanosheet obtained by Firstprinciples calculation.

It obtains the average number (Nc) of adjacent sheets per sheetaccording to the area density of RuO_(2+x) nanosheets. Subsequently, itrandom generate disks in the number (Nc) how many disks, which arepositioned in the left boundary center of square simulation domain (inFIG. 3, at the simulation starting point), are overlapped. Thereby, itselects a disk which is capable of flowing current in the farthest in a+x direction and with the lowest resistance among the generated disks.It random generates disks in the number (Nc) overlapped with theselected disk. The same process is repeated until the selected disk isapproached to the right boundary of the simulation domain. The randomwalk path of the disk is shown in FIG. 3. The random work path is one ofthe current paths of conductive layer including nanosheets. It obtainsthe number of disks providing the path and the total resistance in whichcurrent is flowed in this case. From the results, the equivalent circuitof conductive layer is made to calculate the sheet resistance ofnanosheet conductive layer. The calculation is repeated for severalhundred times to provide an ensemble average, and it estimates the sheetresistance of conductive layer including nanosheet from the ensembleaverage. The estimate is an ideal value that the secondary resistancegenerating factors such as defects are removed and does not consider theresistance which may be caused by polymer or the like used for aconductive layer. The results are shown in FIG. 4.

From the results of FIG. 4, it is confirmed that the micron-sizednanosheets may show a significantly low sheet resistance compared to thesub-micron sized nanosheets.

Without being bound by any particular theory, it is believed that theless influences of contact resistance is made as nanosheets have largerlateral size. In addition, when nanosheet areal coverages are same, thenumber density of sheet is decreased as the lateral size of nanosheetsis increased, so the results may suggest that it may be accumulated intoa sheet resistance of single sheet.

Preparation of RuO_(2+x) Nanosheets Example 1

Ruthenium oxide nanosheets are prepared as schematically shown in FIG.5.

[1] K₂CO₃ and RuO₂ are mixed at 5:8 (mole ratio), and the mixture isformed into pellets. 4 g of the obtained pellet is placed into analumina crucible and heat-treated in a tube furnace at 850° C. for 24hours under the nitrogen atmosphere. The detail heat treatment profileis shown in FIG. 6. The total weight of pellet may be adjusted within arange from 1 to 20 g according to requirements. Subsequently, thefurnace is cooled to a room temperature, and the treated pellet is takenout and pulverized to obtain a fine powder.

The obtained fine powder is agitated and rinsed with about 100milliliters (mL) to 4 liters (L) of water for 24 hours andvacuum-filtered to obtain an intermediate-phase powder. During therinse, an excessive amount of water-soluble impurities such as apotassium compound is removed. The obtained intermediate-phase powderhas a composition of K_(0.2)RuO_(2.1).nH₂O.

The obtained intermediate-phase powder is performed with an inductivelycoupled plasma mass spectrometry (ICP-MS). The results show that K:Ruhas a mole ratio of 0.184:0.814.

The obtained intermediate-phase powder is performed with an X-raydiffraction analysis and a scanning electron microscopic analysis, andthe results are shown in FIG. 7 and FIG. 8. From the results of FIG. 7and FIG. 8, it is confirmed that K_(0.2)RuO_(2.1)nH₂O having a layeredstructure is synthesized. From the results of X-ray diffraction analysisand scanning electron microscopic analysis, it is confirmed that theintermediate-phase powder has an average size of about 123 μm andsubstantially no secondary phase.

[2] The obtained K_(0.2)RuO_(2.1)nH₂O powder is added into 1 molar (M)of HCl solution and agitated for 3 days and filtered to obtain onlypowder. Selectively, the obtained powder may be added into 0.5 M ofH₂SO₄ aqueous solution, again, and agitated for 2 days. The obtainedpowder has a composition of H_(0.2)RuO_(2.1)nH₂O.

1 g of H_(0.2)RuO_(2.1)nH₂O powder is added into 250 mL oftetrabutylammonium hydroxide (TBAOH) aqueous solution and agitated at aroom temperature for 10 days (concentration of TBAOH: TBA+/H+=5).Subsequently, the resulting material is treated with ultrasonic wave ina water bath. The obtained final solution is centrifuged under thecondition at 2000 rpm for 30 minutes to obtain exfoliated RuO_(2+x)nanosheets.

The exfoliated RuO_(2+x) nanosheets are performed with SEM (ScanningElectron Microscopy) analysis, and a part of the results is shown inFIG. 9. From the SEM (Scanning Electron Microscopy) analysis results, itis confirmed that nanosheets have an average lateral length of 1.8 μm.

The obtained nanosheets are performed with a XRD analysis, and theresults are shown in FIG. 10. From the results, it is confirmed that theinterlayer distance is 1.244 nm.

The thickness of obtained nanosheets is measured by an atomic forcemicroscope (AFM). From the results, it is confirmed that the obtainednanosheets have an average thickness of 1.89 nm.

From the results, it is confirmed that the average lateral length ofnanosheets is significantly increased with no substantial increase ofthickness of nanosheets.

Example 2

Exfoliated RuO_(2+x) nanosheets are obtained in accordance with the sameprocedure as in Example 1, except that a mixture of K₂CO₃ and RuO₂ isheat-treated at 850° C. for 48 hours under the nitrogen atmosphere.

During the manufacturing process, the obtained intermediate-phase powderhas a composition of K_(0.2)RuO_(2.1)nH₂O. The obtainedintermediate-phase powder is performed with an inductively coupledplasma mass spectrometry (ICP-MS). From the results, it is confirmedthat K:Ru has a mole ratio of 0.184:0.816.

The obtained intermediate-phase powder is performed with an X-raydiffraction analysis and a scanning electron microscope analysis, andthe results are shown in FIG. 7 and FIG. 11. From the results of FIG. 7and FIG. 11, it is confirmed that K_(0.2)RuO_(2.1)nH₂O having a layeredstructure is synthesized. From the results of the X-ray diffractionanalysis and the scanning electron microscope analysis, it is confirmedthat the hydrate particles have an average size of about 200 μm with nosecondary phase.

The obtained intermediate-phase powder (K shape) and proton-exchangedpowder are performed with XPS analysis, and the results are shown inFIG. 12 and Table 4.

In RuO₂, it is known that Ru⁴⁺ peak is shown in 280.7 to 281.3. From theresults of FIG. 12, it is confirmed that the intermediate-phase powder(K shape) has a Ru3d peak at 281.3 eV; and the proton-exchanged powderhas a Ru3d peak at 281.4 eV. From the results, it is confirmed that Ruhas a tetravalent oxidation number, and Ru3d peak is moved in theproton-exchanged powder due to the high binding energy.

TABLE 4 C1s O1s K2p Ru3p3 KxRuO2.1 4.71 55.61 5.95 33.56 HxRuO2.1 4.7160.99 0.19 33

From Table 4, it is confirmed that the obtained intermediate-phasepowder (K shape) may be represented by K_(0.18)RuO_(2+α). It isconfirmed that almost of K is remove by the protonation.

The exfoliated RuO_(2+x) nanosheets are performed with a SEM analysis,the results are shown in FIG. 13. From the SEM analysis results, it isconfirmed that nanosheets have an average length of 2.3 μm.

The obtained nanosheets are measured for a thickness by an atomic forcemicroscopy (AFM). From the results, the obtained nanosheets have anaverage thickness of 2.46 nm.

Example 3

An exfoliated RuO_(2+x) nanosheets are obtained in accordance with thesame procedure as in Example 1, except that a mixture of K₂CO₃ and RuO₂is heat-treated at 850° C. for 96 hours under the nitrogen atmosphere.

During the manufacturing process, the obtained intermediate-phase powderhas a composition of K_(0.2)RuO_(2.1)nH₂O.

The obtained intermediate-phase powder is performed with an X-raydiffraction analysis and a scanning electron microscopic analysis, andthe results are shown in FIG. 7 and FIG. 14. From the results of FIG. 7and FIG. 14, it is confirmed that K_(0.2)RuO_(2.1)nH₂O having a layeredstructure is synthesized. From the results of the X-ray diffractionanalysis and the scanning electron microscopic analysis, the hydrateparticles have an average size of about 230 microns with no secondaryphase.

The exfoliated RuO_(2+x) nanosheets are performed with a SEM analysis,the results are shown in FIG. 15. From the SEM analysis results, it isconfirmed that nanosheets have an average length of 3.5 μm.

The obtained nanosheets are measured for a thickness by an atomic forcemicroscopy (AFM). From the results, the obtained nanosheets have anaverage thickness of 1.50 nm.

Example 4

An exfoliated RuO_(2+x) nanosheets are obtained in accordance with thesame procedure as in Example 1, except that 1 g of the obtainedH_(0.2)RuO_(2.1)nH₂O powder is added into 250 mL of an aqueous solutionincluding TMAOH and TBAOH (concentrations of TMAOH and TBAOH areTMA+/H+=5, TBA+/H+=5, respectively) and agitated for greater than orequal to about 10 days.

The exfoliated RuO_(2+x) nanosheets are performed with a SEM (ScanningElectron Microscopy) analysis, the results are shown in FIG. 16. Fromthe SEM analysis results, it is confirmed that nanosheets have anaverage length of 7.0 μm.

The obtained nanosheets are measured for a thickness by an atomic forcemicroscopy (AFM). From the results, the obtained nanosheets have anaverage thickness of 1.23 nm.

The obtained nanosheets are performed with a XRD analysis, and theresults are shown in FIG. 10. From the results, it is confirmed that theinterlayer distance is 0.951 nm.

Example 5

RuO_(2+x) nanosheets are obtained in accordance with the same procedureas in Example 2, except that a mixture of K₂CO₃ and RuO₂ is heat-treatedat 850° C. for 48 hours under the nitrogen atmosphere, and 1 g of theobtained H_(0.2)RuO_(2.1)nH₂O powder is added into 250 mL of an aqueoussolution including TMAOH and TBAOH (concentrations of TMAOH and TBAOHare TMA+/H+=5, TBA+/H+=5, respectively) and agitated for greater than orequal to about 10 days.

The exfoliated RuO_(2+x) nanosheets are performed with a SEM (ScanningElectron Microscopy) analysis, and a part of the results is shown inFIG. 17. From the SEM analysis results, it is confirmed that nanosheetshave an average length of 10.2 μm.

The obtained nanosheets are measured for a thickness by an atomic forcemicroscopy (AFM). From the results, the obtained nanosheets have anaverage thickness of 1.50 nm.

Comparative Example 1: Preparation of RuO_(2+x) Nanosheet

An exfoliated RuO_(2+x) nanosheets are obtained in accordance with thesame procedure as in Example 1, except that a mixture of K₂CO₃ and RuO₂is heat-treated at 850° C. for 12 hours under the nitrogen atmosphere.

During the manufacturing process, the obtained intermediate-phase powderhas a composition of K_(0.2)RuO_(2.1)nH₂O.

The obtained intermediate-phase powder is performed with an X-raydiffraction analysis and a scanning electron microscopic analysis, andthe results are shown in FIG. 7 and FIG. 18. From the results of FIG. 7and FIG. 18, it is confirmed that K_(0.2)RuO_(2.1)nH₂O having a layeredstructure is synthesized. From the results of the X-ray diffractionanalysis and the scanning electron microscopic analysis, it is confirmedthat the hydrate particles have an average size of about 60 microns andalso include K₂Ru₈O₁₆ having a needle structure as the secondary phase.

The exfoliated RuO_(2+x) nanosheets are performed with a SEM (ScanningElectron Microscopy) analysis, the results are shown in FIG. 19. Fromthe SEM analysis results, it is confirmed that nanosheets have anaverage length of 0.7 μm.

The obtained nanosheets are measured for a thickness by an atomic forcemicroscopy (AFM). From the results, the obtained nanosheets have anaverage thickness of 3.85 nm.

Preparation of Electrical Conductor Example 6

Prepared is a coating liquid including RuO_(2+x) nanosheets (averagelateral size: 1.8 μm) obtained from Example 1 and having the followingcomposition:

3 g of aqueous dispersion of the obtained RuO_(2+x) nanosheets

0.5 g of HPMC aqueous solution (0.3%)

3 g of isopropanol

1 g of water

The obtained RuO_(2+x) nanosheet coating liquid is bar-coated on apolycarbonate substrate and dried at 85° C. under the air atmosphere.The process is repeated for 3 times to provide a first conductive layer.It is confirmed that the first conductive layer obtained by thebar-coating has a thickness of 1-5 nm. A sheet resistance and a lighttransmittance of the obtained first conductive layer are measured. Fromthe results, it is confirmed that the sheet resistance is 32000 Ω/sq,and the transmittance is 94.4%.

Example 7

A first conductive layer is formed in accordance with the same procedureas in Example 6, except that the RuO_(2+x) nanosheets (average lateralsize: 2.3 μm) obtained Example 2 is used. A sheet resistance and a lighttransmittance of the obtained first conductive layer are measured. Fromthe results, it is confirmed that the sheet resistance is 20250 ohms persquare (Ω/sq), and the transmittance is 94.3%.

Example 8

A first conductive layer is formed in accordance with the same procedureas in Example 6, except that the RuO_(2+x) nanosheets (average lateralsize: 3.5 μm) obtained Example 3 is used. A sheet resistance and a lighttransmittance of the obtained first conductive layer are measured. Fromthe results, it is confirmed that the sheet resistance is 11220 Ω/sq,and the transmittance is 93.9%.

Example 9

A first conductive layer is formed in accordance with the same procedureas in Example 6, except that the RuO_(2+x) nanosheets (average lateralsize: 7.0 μm) obtained Example 4 is used. A sheet resistance and a lighttransmittance of the obtained first conductive layer are measured. Fromthe results, it is confirmed that the sheet resistance is 12625 Ω/sq,and the transmittance is 94.0%.

Example 10

A first conductive layer is prepared in accordance with the sameprocedure as in Example 6, except that RuO_(2+x) nanosheets (averagelateral size: 10.2 μm) obtained from Example 5 is used. A sheetresistance and a light transmittance of the obtained first conductivelayer are measured. From the results, it is confirmed that the sheetresistance is 10250 Ω/sq, and the transmittance is 93.1%.

Comparative Example 2

A first conductive layer is prepared in accordance with the same Example6, except that RuO_(2+x) nanosheets obtained from Comparative Example 1(average lateral size: 0.7 μm) is used. The obtained first conductivelayer is measured for a sheet resistance and a light transmittance. Fromthe results, it is confirmed that the sheet resistance is 87000 Ω/sq,and the transmittance is 94.3%.

The sheet resistances of the conductive layers obtained from Examples 6to 10 and Comparative Example 2 and the average lateral sizes ofnanosheets forming each conductive layer are shown in graph of FIG. 20.From FIG. 20, it is confirmed that the conductive layer includingnanosheets having an lateral size of greater than or equal to about 1.1μm may show the significantly enhanced electrical conductivity (i.e.,decreased sheet resistance) at the similar level of light transmittance.

Example 11

[1] A silver nanowire-containing composition having the followingcomponents is obtained:

3 g of silver nanowire aqueous solution (concentration: 0.5 wt %,average diameter of silver nanowire: 30 nm)

Solvent: 7 g of water and 3 g of ethanol

Binder: 0.5 g of hydroxypropyl methyl cellulose aqueous solution(concentration: 0.3%)

The silver nanowire-containing composition is bar-coated on the firstconductive layer obtained from Example 6 and dried at 85° C. for 1minute under the air atmosphere to obtain an electrical conductor havinga structure of substrate-conductive layer of ruthenium oxidenanosheets/conductive layer of silver nanowire.

Example 12: Evaluation of Flexibility of Electrical Conductor HavingHybrid Structure (Simulation)

[1] The flexibility of the electrical conductor having the hybridstructure is evaluated by the calculation of the silver nanowire randomnetwork sheet resistance based on the following steps:

The silver nanowire random network is made by randomly designating thecentral coordinates (x, y) and angle θ of wire in the square simulationdomain using MATLAB.

For the wires made in the aforementioned manner, it is determinedwhether they meet another wire in the network using a formula forcalculating a distance between two straight lines, and thereby the wirecontact information is stored.

A cluster of wires through which the current may flow from the wirecontacting the left end of square simulation domain to the wirecontacting the right end without stopping is determined using the storedinter-contact information between the wires.

While considering the resistance of Ag NW itself and the contactresistance between the wires for all the wire contact points (junction)in each cluster, a linear equation is established by applying theKirchhoff current law.

In this case, the linear equation is transformed in order to apply a 2Dhybrid wherein the NW junction deformed by bending is a model flowingthrough 2D sheet, and the 2D sheet is assumed to have 100% coverage.

In a delamination model wherein the wire is not cut, it is assumed thatsome of inter-wire junctions are spaced, and current is flowed throughthe 2D sheet instead of the spaced contact and thus the contactresistance of the linear equation corresponding to the spaced junctionis changed to the 2D sheet resistance.

In a broken junction model wherein the wire is cut, the linear equationis transformed by removing the contract resistance of the linearequation corresponding to the broken junction and adding the resistancecorresponding to half of the 2D sheet to the broken wire resistance inseries (referring to FIG. 21). (referred to FIG. 21)

The linear equation is solved as many times as the number of junctionsto calculate the value of the current flowing when a 1 V voltage isapplied from left to right of the square simulation domain, and based onthis current value, the sheet resistance of the Ag NW network iscalculated.

[2] From the results of FIG. 22, it is confirmed that the conductorhaving a nanowire/nanosheet hybrid structure may show an improvedresistance change compared to the conductor having only the nanowire,when the 2D nanosheet layer having predetermined sheet resistance ispresent.

Reference Example 2: Synthesis of Alkali Metal Vanadium Oxide

Na₂CO₃ and V₂O₃ are mixed at 1:1 (mole ratio), and the mixture is formedinto pellets. 4 g of the obtained pellets is placed into a platinumcrucible and heat-treated in a tube furnace at 800° C. for 24 hoursunder the hydrogen/nitrogen (1:9) atmosphere. Subsequently, the furnaceis cooled to the room temperature, and the treated pellet is taken outand pulverized to provide a fine powder.

The X-ray diffraction spectrum of the obtained NaVO₂ is shown in FIG.23. From the results of FIG. 23, it is confirmed that NaVO₂ has alayered structure.

The obtained fine powder may be formed into nanosheets according to thesimilar method to Example 1.

While this disclosure has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A population of nanosheets comprising a pluralityof metal oxide nanosheets, wherein the plurality of metal oxidenanosheets comprise an oxide of Re, V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo,In, Cr, Rh, Mn, Co, Fe, or a combination thereof, and wherein an averagelateral dimension of the plurality of metal oxide nanosheets is greaterthan or equal to about 1.1 micrometers.
 2. The population of nanosheetsof claim 1, wherein the plurality of metal oxide nanosheets has anaverage lateral dimension of greater than or equal to about 1.5micrometers, and an average thickness of less than or equal to about 5nanometers.
 3. The population of nanosheets of claim 1, wherein theplurality of metal oxide nanosheets comprises RuO_(2+x) wherein 0≤x≤0.5,MnO₂, Mn₃O₇, Mn_(1−x)Co_(x)O₂ wherein 0<x≤0.4, VO₂, CoO₂, FeO₂, ReO₂,IrO₂, InO₂, or a combination thereof.
 4. A method of preparing thepopulation of the nanosheets according to claim 1, the method comprisingheat-treating a mixture comprising a transition metal oxide comprisingRe, V, Os, Ru, Ta, Ir, Nb, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or acombination thereof and an alkali metal compound at a temperature ofabout 750° C. to about 950° C. for about 18 hours or more to obtain alayered alkali metal-transition metal oxide; pulverizing the layeredalkali metal-transition metal oxide to obtain a powder of the layeredalkali metal-transition metal oxide; rinsing the powder of the layeredalkali metal-transition metal oxide with water to obtain a powder of alayered alkali metal-transition metal oxide hydrate; treating the powderof the layered alkali metal-transition metal oxide hydrate with anacidic solution to obtain a layered proton-exchanged transition metaloxide hydrate wherein at least a portion of an alkali metal is exchangedwith a proton; contacting the layered proton-exchanged transition metaloxide hydrate with a C1 to C16 alkyl ammonium salt compound to obtain alayered transition metal oxide intercalated with a C1 to C16 alkylammonium cation; and mixing the layered transition metal oxideintercalated with the C1 to C16 alkyl ammonium cation with a solvent toobtain a population of transition metal oxide nanosheets.
 5. The methodof claim 4, wherein the heat-treating is performed for about 24 hours ormore.
 6. The method of claim 4, wherein the powder of the layered alkalimetal-transition metal oxide hydrate has an average particle diameter ofgreater than or equal to about 100 micrometers.
 7. The population ofnanosheets of claim 1, wherein an average thickness of the plurality ofmetal oxide nanosheets is less than or equal to about 5 nanometers. 8.The population of nanosheets of claim 1, wherein the average lateraldimension of the plurality of metal oxide nanosheets is greater than orequal to about 1.8 micrometers.
 9. The population of nanosheets of claim1, wherein the average lateral dimension of the plurality of metal oxidenanosheets is greater than or equal to about 2.3 micrometers.
 10. Thepopulation of nanosheets of claim 1, wherein the average lateraldimension of the plurality of metal oxide nanosheets is greater than orequal to about 3.5 micrometers.
 11. The population of nanosheets ofclaim 1, wherein the average lateral dimension of the plurality of metaloxide nanosheets is greater than or equal to about 7.0 micrometers. 12.The population of nanosheets of claim 1, wherein the average lateraldimension of the plurality of metal oxide nanosheets is less than orequal to about 30 micrometers.
 13. The population of nanosheets of claim1, wherein the plurality of metal oxide nanosheets comprise an oxide ofRe, V, Os, Ru, Ir, W, Ga, Mo, In, Cr, Rh, Mn, Co, Fe, or a combinationthereof.
 14. The population of nanosheets of claim 1, wherein theplurality of metal oxide nanosheets comprise at least two types ofalkylammonium compounds.
 15. The population of nanosheets of claim 14,wherein the at least two types of alkylammonium compounds comprises atleast one of tetramethylammonium compound and tetraethylammoniumcompound and at least one of a tetrapropylammonium compound, abenzylalkylammonium compound and a tetrabutylammonium compound.
 16. Acomposition comprising the population of nanosheets of claim 1 and asolvent.
 17. The composition of claim 16, wherein the solvent compriseswater and optionally an alcohol.
 18. The composition of claim 16,wherein the composition further comprises a dispersing agent.
 19. Thepopulation of nanosheets of claim 1, wherein the average lateraldimension of the plurality of metal oxide nanosheets is less than orequal to about 10.5 micrometers.